Use a quad composition layer object from osgXR 0.3.9 to make the splash
screen VR friendly. This allows the OpenXR compositor to redraw the quad
without any updates from flightgear, which is particularly helpful as
flightgear makes no attempt at a reasonable VR frame rate during
initialisation.
The quad is positioned 2 meters from the user, with an aspect ratio
matching the desktop window. The quad is blended using an
unpremultiplied alpha channel which is forced to a given alpha value by
osgXR.
The splash is effectively already rendered to an FBO as non-linear SRGB,
so this is made explicit in the internal texture format, with the
rendering to the desktop window using GL_FRAMEBUFFER_SRGB to ensure it
is properly re-encoded.
Using a <condition> images can be hidden now.
Also reworked the logic so that any hidden item will be hidden from creation and not rely on the update logic; as otherwise items can momentarily appear.
Refactor Splash to be data drive using definitions in <content> for the system splash and <model-content> for model related content.
All system splash is at fixed index and should not be changed except for good reason.
Uses TTF fonts, and displays more information textually including
the application version and current aircraft.
Also rename FGRenderer::splashinit to preinit, as was suggested
a long time ago.
